。
Science is what she's worst at.
Literal
She [topic-は] science [subject-が] most weak-at is.
~が苦手 is the standard 'bad at / weak with X' pattern. 苦手 is a na-adjective and a state predicate, so the thing she's bad at is marked with が — same shape as ~が好き, ~が上手. Adding 最も ('most') turns it into a superlative: 'science is what she's worst at, of everything.' 苦手 is gentler than 嫌い ('dislike'); it frames the issue as personal incapacity rather than active rejection — useful when you want to be tactful about someone's weak point.